Real estate brokerage company Compass has filed a lawsuit against Zillow, claiming the popular real estate website is refusing to accept home listings for properties that have been initially advertised for sale elsewhere.

Compass, the real estate brokerage, claims that Zillow, the website that has 227 million unique visitors a month, has conspired to maintain a monopoly over digital home listings.

Zillow’s listing standards policy is slated to go into effect on Monday, but not if Compass gets its way with the court.